Ultrasonic scanning heads are used in the measuring and testing of metal structures. The scanning heads are relatively small and may be hand held and include a transducer and one or more rollers, rolling over the surface being tested. A scanning head and acoustical imaging system useful in imaging thickness and defects in structures such as tanks or pipelines is shown in Rutherford U.S. Pat. No. 4,625,557. In order to limit the acoustical impedance of air, the area between the transducer and the metal surface being tested or imaged is flooded with a couplant. An example of such a system is shown in copending application of Larry A. Weigel, Ser. No. 08/317,348, filed Oct. 4, 1994 and entitled "Scanner Head Assembly And Couplant System Therefor". This makes the work site wet and messy, for example. Also, the work is often done outdoors or in less than ideal environments. Acoustical imaging underwater is very effective with the systems described, except for one major problem.
The optical encoder driven by the roller is sensitive to the environment, and underwater or in other deleterious environments, the optical encoder tends to fail. Optical encoders are not what could be considered spare parts which would be handy at the test site. Even if readily available, they are not easy to replace. Failure of the encoder brings the entire project to a halt until a new one can be shipped and the failed one replaced.
Environmentally isolated optical encoders are not commercially available. Attempts to encase the encoder with the drive shaft projecting from the encasement and sealed to prevent water ingress have been less than satisfactory. Even the best of rotating seals tends to fail due to the severe environmental conditions encountered, particularly in underwater work. Such work encounters salt, sand, grit and general junk, for example, all of which adversely affects rotating seals. With the encoder encased or encapsulated, the failed encoder resulting from a failed seal is even more difficult to repair or replace.
It would, accordingly, be desirable to have a scanning head with an encoder which is completely physically isolated from its environment including its drive means, so that there are no wear sensitive points or areas which might permit environmental ingress to the encoder.